Variability in renal tubular flow rates subject tubular epithelial cells to changes in shear stress and hydrostatic pressure that ultimately affects cellular function. The distal nephron, and specifically the cortical collecting duct (CCD), is comprised of 2 major cell types: Na-absorbing principal cells (70%) and acid-base transporting intercalated cells (30%). Principal cells possess apical epithelial Na channels (ENaCs), which have a key role in transepithelial Na absorption. Acid-base transport by intercalated cells is mediated by apical anion exchangers and proton pumps localized to beta-and alpha-intercalated cells, respectively. Rabbit CCDs respond to an increase in flow with an increase in Na absorption as well as a reduction in bicarbonate secretion. This application will address mechanisms underlying flow-dependence of ENaC activation, and thus extend studies begun in the current funding period, and initiate an investigation directed at exploring mechanisms underlying flow-regulation of proton and bicarbonate transport. Proposed studies will utilize CCDs, cultured epithelial cells and Xenopus oocytes to determine mechanisms by which flow increases ENaC open probability. Studies in rabbit CCDs will address mechanisms by which flow reduces net bicarbonate secretion. These proposed studies should provide new information regarding the regulation of Na and acid/base transport in the CCD.